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Classes for the creation of undoable operations which can be added to an operations
history and later be undone and redone.
<h2>
Package Specification</h2>
An <strong>undoable operation</strong> is a unit of work that can be executed, undone, and redone.
Operations can be added to an <strong>operation history</strong> so that they can later be
undone and redone according to the undo model for an application.  Operations may be
assigned one or more <strong>undo contexts</strong> which can be used to filter the available
operations for undo or redo in the operation history.  Clients may choose to provide
undo and redo function for all operations in a history, or only for a particular
undo context in that history.  Operation histories may be configured with an
<strong>operation approver</strong> so that applications can enforce any desired undo model,
such as strict linear (LIFO) undo.  This package provides the definition and a
basic implementation for operations, undo contexts, histories, and operation
approvers.
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